Tail gas from Sulfur Recovery Units for acid gas contains a variety of sulfur compounds, which must be destroyed by combustion before the gases are vented to the atmosphere. The units can be natural draft or forced draft operated. Oxidizing temperatures can vary from 1200 to 1800° F with residence times of 0.6 to 1.0 seconds. The equipment generally consists of a burner and a castable refractory lined vertical thermal oxidizer/stack combination unit from100ft to greater than 300ft in height. An external rain shield is invariably provided to maintain a hot stack steel shell to prevent dew point condensation and resulting corrosion. Alternatively, a mastic liner may be used as a barrier between the flue gases and the inside of the steel shell. Regulations require H2S in the flue gas be generally limited to 10 ppmV but can require as low as 5 ppmV in certain cases. CO coming in with the Tail gas may not be destroyed at 1200° F, however, a substantial portion will at 1400° F and above.